What is the reference Nixpkgs?

Users can include their repo's with expressions. User A might build and test against 18.03, whereas user B tests against master, and user C something else. So, what should be the reference Nixpkgs? Or should there not be any?

I thought, I have documented that, but apparently not. For the start we should target master as the project is still subjected to changes in its structure. How stable releases are targeted is something we need to figure out later.